I often get buds bleached by the lights and burnt yellow leafs closest to the light. The light mover has solved this problem. On top of that the constantly changing angle of light better penetrates the canopy. Wish I started using a light mover years ago.
Ppl use movers for different reasons. ODB and I are thinking along the same lines here. I use the mover out of necessity more than anything. In my very narrow space, a stationary 600 would not be good for any part of my canopy within 12" of said bulb. But with the mover, the buds are often within less than 6" of the bulb for short periods of time. With out the mover, I'd have to go back to 4oo watt bulbs.
Light movers don't do much when u have bulbs stacked already. They do cause a lot of headaches and another spot for something to go wrong.
So MM, as stacked lights may work for you, and several others, they are not ideal for maximizing every situation.